During the processing of powder and granular shaped bulk materials, for example, in the food or pharmaceutical industries, it often becomes necessary to free these bulk materials from unwanted foreign bodies. It is especially important to eliminate these foreign bodies from products which are intended to be used for human consumption. These foreign bodies originate at various sources. One often might find stones which were picked up at the harvest of grain or other agricultural products. At other times, there may be nutshells which were not entirely removed when the nuts were shelled. Under certain circumstances, one could even find screws, nuts and bolts which, during the transport of the bulk material, found their way into the bulk material. Accordingly, it has been necessary to strain bulk materials to remove these foreign bodies before additional processing can be performed. To accomplish this, a strainer or screen with the required mesh size has been used. The material is placed in the strainer and through motions of the strainer, perhaps a vibration or movement of the material above the strainer, e.g., by using a main beater mechanism, a portion of the bulk material which has the necessary fineness, will move through the strainer. The foreign bodies are retained by the strainer and can be removed. Of course, it is possible that heavier foreign bodies could hit the strainer with such an impact force that the strainer might get damaged. When this happens, the foreign bodies will wind up with the bulk material that was supposed to be designated as the cleaned portion. Thus, after the strainer is damaged, the resulting yield of the straining device will not be maintained in the required quality range.
U.S. Pat. No. 4,202,759 discloses a straining device of the type discussed above and includes an additional screen. This additional screen serves simultaneously as a carrier for a beater mechanism. That is, a mechanism which causes the relative motion of the material to be strained. The screen rotates with respect to the main screen with a rotational speed of approximately 700 r.p.m., thereby causing the bulk material to be strained through the additional screen, mainly by centrifugal force. Although the additional screen does provide the benefit of keeping larger foreign bodies away from the main strainer, there is a tendency for certain bulk materials to clog. This is true with many powder and granular materials, especially if they are exposed to a certain amount of humidity. In many cases, an incorrect control setting of the air humidity in the atmosphere surrounding the strainer will cause the formation of lumps in the bulk material. Once the material has formed lumps, it can no longer be strained through the additional screen by centrifugal force alone and this additional screen will quickly clog up, thereby preventing the further use of the strainer without maintenance.